Without having a go in any of the GT racing cars I couldn't accurately compare it to sims that specialise in GT and racing cars. Road cars and racing cars do not feel the same to drive and in Prologue we only had road cars and tuned road cars, no purpose built race cars, unless the Ford GT LM was supposed to be one but we don't really know.
I would go out on a limb though and say that GT5 is better than a lot of PC sim elitists give it credit for. Sure it falls short of iRacingg, but I think it's up there. Not the best, and there will be areas other sims are better as I'm sure areas where GT5 bests thoes. If iRacing was 100% then I'd put GT5 at between 85-90%. It real life was 100% then I'd put it much, much lower, there's still far too much you can't simulate in a computer game.